The invention relates to an electric lamp comprising a light-transmitting lamp vessel which accommodates a light source, at least a part of the lamp vessel being provided with an interference film, the interference film including layers of alternately a first layer which is made predominantly of silicon oxide and a second layer which is made of a material having a refractive index which is high as compared to a refractive index of silicon oxide which is 1.45.
An electric lamp of this type is known from U.S. Pat. No. 5,138,219. In the known discharge lamp, the layers having a high refractive index are made of tantalum oxide. These layers may alternatively consist of niobium oxide or titanium oxide. The layers may also be composed of a combination of two layers, for example TiO2.ZrO2, TiO2.HfO2, TiO2.Nb2O5, TiO2.Ta2O5 or Ta2O5. 2TiO2.
It is a drawback of lamps having an interference film on the lamp vessel that the film is exposed to substantially varying temperatures (above 500xc2x0 C.). As a result, diffuse scattering of the interference film during the service life of the electric lamp increases. Diffuse scattering leads to an unclear transparent lamp. Diffuse scattering additionally causes the direction in which (for example infrared) radiation is reflected by the interference film to be changed relative to the desired direction, thus causing the efficacy of the lamp to be reduced.
It is an object of the invention to provide an electric lamp, in which diffuse scattering of the interference film is reduced.
In accordance with a first aspect of the invention, one of the second layers comprises an odd number of sub-layers of alternately a first sub-layer of the material with the high refractive index (over 1.45) and a second sub-layer of a further material with a high refractive index (over 1.45).
During the service life of the electric lamp, the lamp vessel of which is provided with an optical interference film, crystallization occurs of, in particular, the (high-refraction) material of the second layer of the interference film. In addition, as a result of temperature changes, the crystallization state of the material of the second layer may change. Crystallization is regarded as one of the causes of diffuse scattering occurring during the service life of a lamp provided with a lamp vessel covered with an interference film. It has been found that the crystals can grow bigger as the thickness of the layer of the high-refraction material is larger. The effect on diffuse scattering increases as the crystals become bigger. If, in accordance with a first aspect of the invention, the (relatively thick) second layers of the high-refraction material in the interference film are built up of a stack of an odd number of sub-layers, whereby between two first sub-layers of the material with the high refractive index a second sub-layer of a further material with a further high refractive index is sandwiched, the possibilities of crystal growth in the second layer are effectively reduced by 50%. By virtue of the measure in accordance with the invention, crystals developing in the odd sub-layers can only grow within said sub-layer and will consequently remain relatively small, so that the effect of such crystals on diffuse scattering is reduced. By sandwiching a (second) sub-layer of the further material with the further high refractive index between two (first) sub-layers of the material with the high refractive index, the relatively thick layer of the material with the high refractive index is interrupted as it were, which has a favorable effect on the reduction of undesirable diffuse scattering.
Preferably, the further material of the second sub-layer is selected from the group formed by tantalum oxide, zirconium oxide, hafnium oxide and combinations of these materials. Particularly the high-refraction optical material tantalum oxide exhibits favorable properties at higher temperatures. It is further desirable that the refractive index of the further material forming the second sub-layer should correspond at least substantially to the refractive index of the material of the first sub-layer. If so, the spectrum of the interference film in accordance with the invention hardly differs from that of the known interference film. A suitable choice of the material of the second sub-layer (for example a combination of two high-refraction materials) enables the refractive indices of the materials of the first and the second sub-layer to be properly matched. Suitable combinations of high-refraction materials are TiO2.ZrO2, TiO2.HfO2 and TiO2.Nb2O5. A further suitable combination of high-refraction materials is TiO2.Nb2O5. In a very favorable, alternative embodiment of the electric lamp in accordance with a first aspect of the invention, the further material of the second sub-layer includes a combination of titanium oxide and tantalum oxide. Such a combination of materials (for example TiO2.Ta2O5 or Ta2O5.2TiO2) unites the favorable property that the refractive index of titanium oxide is relatively high relative to that of silicon oxide with the favorable behavior of tantalum oxide at relatively high temperatures.
Preferably, the stack of an odd number of alternately a first and a second sub-layer comprises three sub-layers, namely two (first) sub-layers of the high-refraction material between which a (second) sub-layer of the further material with the further high refractive index is sandwiched.
In accordance with a second aspect of the invention, one of the second layers comprises an odd number of sub-layers of alternately a first sub-layer of the material with the high refractive index and a second sub-layer of silicon oxide, the optical layer thickness dop of the second sub-layer lying in the range 1xe2x89xa6dopxe2x89xa620 nm.
By incorporating such a second sub-layer of silicon oxide having a relatively low refractive index, the optical layer thickness of which is chosen so that these second sub-layers contribute little, or not at all, to the optical effect of the interference film, the possibilities of crystal growth in the second (high-refraction) layer are effectively reduced by 50%. By virtue of the measure in accordance with the invention, crystals which develop in the odd sub-layers can grow only within said sub-layer and, as a result, will remain relatively small, so that the effect of such crystals on diffuse scattering is reduced. By sandwiching a (second) sub-layer of silicon oxide between two (first) sub-layers of the material with the high refractive index, the relatively thick layer of the material with the high refractive index is interrupted as it were, which has a favorable effect on the reduction of undesirable diffuse scattering.
An expression which in connection with the term optical layer thickness is known to those skilled in the art is QWOT (=xe2x80x9cQuarter Wave Optical Thicknessxe2x80x9d), which is defined as the wavelength at which the optical thickness of a layer is equal to a quarter (0.25) of the design wavelength of the stack of the interference film, that is:
QWOT=4ndph cos xcex1
where nxc3x97dph is the product of the (complex) refractive index n and the physical layer thickness dph, and xcex1 is the angle at which the light is incident on the interference film (if light is incident transversely to the stack of layers, then xcex1=0xc2x0). As a result of the relative simplicity of stacks of such so-called xe2x80x9cquarter-wavexe2x80x9d optical layer thicknesses, designs of interference films are often referred to in terms of fractions of xe2x80x9cquarter-wavesxe2x80x9d at a reference wavelength.
Preferably, the optical layer thickness of the second sub-layer is smaller than or equal to 10 nm (dopxe2x89xa610 nm). Such relatively thin sub-layers with such a small optical layer thickness practically have no effect on the spectral characteristic of the interference film. An existing design of an interference film does not have to be adapted if the optical layer thickness of the second sub-layer is smaller than or equal to 5 mn (dop less than 5 nm).
Preferably, the stack of an odd number of alternately a first and a second sub-layer comprises three sub-layers, that is two (first) sub-layers of the high- refraction material between which a relatively thin (second) sub-layer of silicon oxide with a relatively low refractive index is sandwiched.
It has been found that the interference film of the electric lamp in accordance with the invention keeps its initial properties to a large extent during the service life of the lamp.
In a favorable embodiment of the electric lamp in accordance with the invention, the physical layer thickness dph of the second sub-layer lies in the range 1xe2x89xa6dphxe2x89xa625 nm. If the thickness of the second sub-layer exceeds 25 nm, then there is a substantial influence on the optical properties of the interference film, so that undesirable side-effects on the transmission or reflection properties of the interference film occur. A lower limit of 1 nm is desirable to ensure that at least one or a few monolayers of the second sub-layer are situated between the first sub-layers.
Preferably, the physical layer thickness dph of the second sub-layer lies in the range 2 less than dph less than 12 nm. The effect of such layer thicknesses on the spectral characteristic of the interference film is small. Physical layer thicknesses of the second sub-layer below or equal to 5 nm (dph less than 5 nm) are particularly favorable.
Preferably, the material of the first sub-layer comprises predominantly niobium oxide or titanium oxide. Of the high-refraction niobium oxide (Nb2O5) various crystal forms are known: while in the course of a sputtering process an amorphous material is deposited, this material exhibits, at approximately 300xc2x0 C., a transition to a so-called T-structure with a substantially hexagonal cation lattice. At approximately 500xc2x0 C., a transition to a so-called TT-structure takes place. At approximately 700xc2x0 C., a slow transition to a so-called H-structure, which causes diffuse scattering, occurs. Of the high-refraction titanium oxide (TiO2) two crystal forms are known (anatase and rutile) which blend with each other at approximately 600xc2x0 C. Particularly the high-temperature rutile form causes diffuse scattering. If said high-refraction materials (Nb2Ob and TiO2) are incorporated in an interference film on a lamp vessel of an electric lamp, which lamp vessel during the service life of the lamp is subject to (frequent) temperature changes between room temperature and temperatures above 500xc2x0 C. (temperatures as high as 900xc2x0 C. are no exception), these materials are known to cause diffuse scattering of the interference film, thereby adversely affecting the action of the interference film. Particularly for materials of the second layer, such as niobium oxide or titanium oxide, the measure in accordance with the invention can be advantageously used.
The light source of the lamp may be an incandescent body, for example in a halogen-containing gas, but it may alternatively be an electrode pair in an ionizable gas, for example an inert gas with metal halides, if necessary with, for example, mercury as the buffer gas. The light source may be surrounded by an innermost gastight envelope. It is alternatively possible that an outermost envelope surrounds the lamp vessel.
The interference film may transmit visible light and reflect IR (=infrared) light. In this case, the layers of the film are relatively thick, the optical layer thickness being a quarter of a wavelength in the IR-region of the spectrum. It is alternatively possible for the film to reflect UV (=ultraviolet) light and transmit visible light, or conversely. The filter may also be a so-called xe2x80x9cbandpassxe2x80x9d filter, which transmits a part of the visible radiation and reflects other parts of the spectrum. In the case of an IR-reflecting filter having relatively thick layers, and also if a relatively large number of layers are stacked and the interference film has a relatively large thickness of, for example, 3.5 to 4 xcexcm, the interference film in accordance with the measure of the invention was found to retain its favorable properties.
The interference film may be provided in a customary manner, for example by vapor deposition (PVD: physical vapor deposition) or by (DC) (reactive) sputtering or by means of a dip-coating process or LP-CVD (Low Pressure Chemical Vapor Deposition), or PE-CVD (Plasma Enhanced CVD).
These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter.